Bioimaging research has focused considerable attention on near-infrared (NIR) dyes. Imaging in the NIR region (approximately 650 nm to approximately 1000 nm) enables deeper tissue penetration with lower auto-fluorescence background than does imaging in the visible range, and is, therefore, particularly suitable for in vivo studies. With the emerging interest in small animal in vivo imaging, there is an increasing need for novel NIR fluorescent probes that exhibit good photostability.
Surface-enhanced Raman spectroscopy (SERS) is an alternative to fluorescence-based spectroscopy in bioimaging, as it can minimize photobleaching, peak overlapping, and signal-to-noise ratios in complex biological systems. SERS probes take advantage of the 1014-1016-fold scattering enhancement caused by the proximity of Raman-active molecules to the surface of metal nanoparticles. However, most of the commonly used Raman-active molecules are active in the ultraviolet (UV)-visible range, and thus have restricted potential for in vivo imaging. The advantages of imaging in the NIR region have increased interest in NIR SERS-active molecules. Although 3,3′-diethylthiatricarbocyanine (DTTC) is used as a standard in NIR SERS studies, it has only a moderate Raman intensity. There is, therefore, a need for ultra-sensitive SERS probes for use in the NIR region.